This invention relates to treatment of BOD-containing wastewaters, such as municipal sewage, industrial wastewaters and the like, by an activated sludge process and, more particularly, to such a process wherein a wastewater influent and recycled activated sludge are first mixed in an aeration zone into which an oxygen-containing gas is introduced.
Activated sludge processes have been used for some time to remove biological oxygen demand (BOD) from municipal sewage, industrial wastewaters and the like. In such a process, a wastewater influent is mixed with a microorganism-containing recycled biomass or activated sludge in an initial contact zone to form a mixed liquor. At some point in the process the mixed liquor is aerated with sufficient oxygen to grow and maintain a satisfactory population of microorganisms which sorb, assimilate and metabolize the BOD of the wastewater.
In the activated sludge process disclosed in U.S. Pat. No. 4,056,465, the wastewater and recycled activated sludge are mixed with mechanical stirring means in an initial contact zone from which air is excluded in order to operate under anaerobic conditions. The mixed liquor is subsequently treated in one or more aeration or aerobic zones before being clarified to separate an activated sludge.
In the so-called Bardenpho activated sludge process disclosed in U.S. Pat. No. 3,964,998, wastewater and recycled activated sludge are mixed with mechanical stirring means in a first stage which is operated under anoxic conditions. The mixed liquor is subsequently aerated in a second stage, subjected to anoxic conditions in a third stage, aerated in a fourth stage and then clarified to separate an activated sludge.
In a so-called orbal process, wastewater and recycled reactivated sludge are mixed and circulated around a plurality of concentric, annular basins or channels by a plurality of surface aeration discs which churn oxygen into the upper surface of the mixed liquor and provide sufficient agitation to prevent settling. The mixed liquor flows from one channel to the next and finally is introduced into a clarifier to separate an activated sludge. The channels can be operated as a series of complete mix reactors so that the dissolved oxygen content in the first channel in which the wastewater and recycled activated sludge is initially mixed is about 0 and the dissolved oxygen content is subsequently increased as the mixed liquor moves from one channel to the next.
The mechanical stirring and aeration devices required by these processes involve considerable equipment and operating costs. Also, in some systems it is difficult to control the dissolved oxygen-content within a desired range when many of these aerating devices are used as a means for introducing oxygen into the first aeration zone.
Other activated sludge wastewater treatment processes are disclosed in U.S. Pat. Nos. 3,764,523, 3,939,068, 3,953,327 and 3,994,802.
Applicants are unaware of any prior activated sludge wastewater treatment processes wherein wastewater and recycled activated sludge are initially mixed in an aeration zone which is operated as a complete mix reactor and at a low dissolved oxygen content and the source for the oxygen-containing gas also serves as the sole, or at least the primary, source of agitation for the mixed liquor.